There are many different arc welding processes used for numerous welding applications. While different processes share some characteristics, such as using an electric arc and/or current flow to provide the heat for the weld, different processes have characteristics that render them desirable for particular applications.
MIG welding is a widely used process that gives high heat input into the wire electrode and the workpiece, and thus can give high deposition rates. However, the process can be unstable and control of the arc length can be difficult. The MIG process is often performed as a short circuit welding.
Another known welding process is called controlled short circuit welding, or short circuit welding. Short circuit welding is often performed as a MIG process. Generally, short circuit welding includes a short circuit state, wherein the welding wire is touching the weld pool thus creating a short circuit, and an arc state, wherein an arc is formed between the welding wire and the weld pool. During the arc state the wire melts, and during the short circuit state the molten metal is transferred from the end of the wire to the weld puddle.
Disadvantages of short circuit welding relate to the transitions between states, and instability of the process. Transition from the short circuit state to the arc state was typically caused by providing sufficient current to “pinch” off a droplet. The pinching off at high current can result in a violent disintegration of the molten metal bridge producing excessive weld spatter. Instability also results from the weld pool being pushed away.
Many attempts in the prior art were made to create a stable short circuit welding power supply, such as those shown in U.S. Pat. Nos. 4,717,807, 4,835,360, 4,866,247, 4,897,523, 4,954,691, 4,972,064, 5,001,326, 5,003,154, 5,148,001, 5,742,029, 5,961,863, 6,051,810 and 6,160,241. These patents generally disclose complicated control schemes that fail to control the process to provide a stable and effective weld. They include control schemes that try to control the deposition of material and/or predict or cause a transition to the subsequent state based on the total energy put into the weld, the length of the stick out, total watts, time of the preceding state, etc.
These schemes share a common failure: they attempt to control both the energy of the weld and the transition between states using output current or power. This necessarily entails a sacrificing of one control goal (either energy to the weld or state transition) for the sake of the other. The net result is that the control schemes do not perform well at either controlling the energy into the weld or controlling the transition.
Another short circuit welding control system is disclosed in U.S. Pat. No. 6,326,591. This system adequately controls the energy into the weld, but it does not provide independent control of the transitions between states.
The present inventors have published descriptions of a controlled short circuit welding process where mechanical movement of the wire (advancing and retracting) is used to control the transition between welding states. The short circuit state is entered by advancing the wire until the wire touches the weld pool. The arc state is entered by retracting the wire until the wire does not touch the weld pool, and an arc forms. This system allows a typical output control to be used to control the energy delivered to the weld. By separating control of the transitions from control of energy, the system allows for better control of each.
A controlled short circuit welding system requires the capability of advancing and retracting the wire. The inventors have disclosed in the literature the use of a stepper motor to control the wire movement. A stepper motor adequately provides for short term advancing and retracting of the wire.
However, a stepper motor does not necessarily provide adequate feeding of the wire over the long term. Accordingly, a system that provides for advancing and retracting of the wire, and long term feeding of the wire, is desirable.
One problem with controlled short circuit welding arises when the wire is retracted. The wire from the source is feeding toward the weld, and has momentum in that direction. The retracting motor moves the wire in the opposite direction. With nothing to compensate for the opposing forces, the wire might not feed in a smooth and efficient manner. Accordingly, a controlled short circuit welder that compensates for the reversal of the wire is desirable.
Another problem with controlled short circuit welding is that the prior art has not fully taken advantage of the process control made possible by the mechanical control of the state transitions. Thus, a controlled short circuit welder that provides for electrical control of the arc for the purpose of controlling heat into the weld, and not for causing transitions from one state to another, is desirable.
The prior art has not adequately addressed the needs of short circuit welding at lower currents with thicker wires. The difficult to implement control schemes, in particular, make it difficult to weld with thicker wire, such as 2.4 mm diameter wire, e.g., at low currents, such as less than 100 amps. Accordingly, a controlled short circuit welding process that may be used at low currents relative to the wire diameter is desirable.